This invention relates to a vapor-deposited film of selenium or selenium alloy as a photoreceptor for electrophotography and also to a process for producing the same, characterized in that the contrast potential of the resulting photoreceptor is controlled by doping the selenium or selenium alloy with phosphorus.
Electrophotography is a photocopying process which utilizes the photoconductivity and electrostatic behavior of certain substances. Of some methods so far established, one known as the Xerox process, or xerography, involves the use of a vapor-deposited film of selenium as a photoreceptor and the transfer of image for printing as an electronic photograph. The process comprises the steps of:
(a) charging, in which the surface of a photosensitive plate, consisting of an amorphous selenium layer with a high dark resistance vapor-deposited on a metal substrate, is electrostatically charged; PA1 (b) exposure (printing), in which, upon exposure to an optical image, the areas of selenium irradiated with the light undergo a decrease in the electric resistance, the electrostatic charge escapes from the surface to the metal substrate, and the residual charge density on the photosensitive plate varies according to the amounts of exposure, thus forming an electrostatic latent image of the same pattern as the original copy on the selenium surface; PA1 (c) development, in which a mixed powder of a toner, consisting of fine resin-coated carbon particles, and a carrier, consisting of microspheroidal glass, is dusted over the photosensitive plate surface, so that the toner adheres to the latent image and makes it visible; PA1 (d) transfer, in which a sheet of appropriate paper is placed over the developed photosensitive plate surface, and the back of the paper is charged by the use of a corona discharge, whereby the toner on the photosensitive plate is attracted to the paper and the toner powder image is transferred onto the paper; and PA1 (e) fixing, in which the paper onto which the image has been transferred is removed from the photosensitive plate surface and is heated by an infrared heater to fuse and deposit the toner resin permanently onto the paper. PA1 (1) enhance the sensitiveness of selenium in the long-wave range, PA1 (2) reduce the residual potential toward zero, and PA1 (3) improve the mechanical strengths and wear resistance.
Through the practice of a sequence of these steps a reproduced image (electronic photograph) of the original is obtained. Clearness of the copied image, or the reproducibility of the original, is largely dependent on the performance of selenium as the photoreceptor. For the evaluation of the photoreceptor performance, apparently useful measures include: (i) the corona charge characteristic which represents the quantity of the electrostatic charge produced by a corona discharge of a given output; (ii) the dark decay characteristic related to the loss of charge while the photoreceptor, charged by the corona discharge, is held in the dark; (iii) charged potential decrease characteristic on exposure which represents the rate at which the charge held in darkened conditions disappears upon exposure; and (iv) residual potential which represents the potential that remains, instead of reduction to naught, after the exposure of the photoreceptor. For the adoption of selenium as a photoreceptor for a copying machine the color tone of the copy is an important determinant. The tone, in turn, depends to a large measure on the contrast potential, or the potential difference between the surface exposed to light and the non-exposed surface. For the tone control, therefore, control of the contrast potential, or control of the residual potential in the exposed region, is important.
Attempts have been made to add varied impurities to selenium in order to improve its electrophotographic properties. The impurities reported to have hitherto been added for that purpose are Te, As, Si, Sb, halogens, Cu, Ag, Zn, and the like. The additives have been primarily intended to:
The prior attempts have, however, failed to control as desired the contrast potential of the selenium photoreceptor by the addition of such impurities.